Investigations of thiol-modified phenol derivatives for the use in thiol-ene photopolymerizations

Article abstract of DOI:10.3762/bjoc.10.180

Thiol-ene photopolymerizations gain a growing interest in academic research. Coatings and dental restoratives are interesting applications for thiol-ene photopolymerizations due to their unique features. In most studies the relative flexible and hydrophilic ester derivative, namely pentaerythritoltetra(3-mercaptopropionate) (PETMP), is investigated as the thiol component. Thus, in the present study we are encouraged to investigate the performance of more hydrophobic ester-free thiol-modified bis-and trisphenol derivatives in thiol-ene photopolymerizations. For this, six different thiol-modified bis-and trisphenol derivatives exhibiting four to six thiol groups are synthesized via the radical addition of thioacetic acid to suitable allyl-modified precursors and subsequent hydrolysis. Compared to PETMP better flexural strength and modulus of elasticity are achievable in thiol-ene photopolymerizations employing 1,3,5-triallyl-1,3,5-triazine-2,4,6-trione (TATATO) as the ene derivative. Especially, after storage in water, the flexural strength and modulus of elasticity is twice as high compared to the PETMP reference system.

Full text of DOI:10.3762/bjoc.10.180

Investigations of thiol-modified phenol derivatives for the use
in thiol–ene photopolymerizations
Sebastian Reinelt1, Monir Tabatabai1, Urs Karl Fischer2, Norbert Moszner2,
Andreas Utterodt3 and Helmut Ritter*1
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2014, 10, 1733–1740.
1Heinrich-Heine-University Düsseldorf, Institute of Organic Chemistry
and Macromolecular Chemistry, Department of Preparative Polymer
Chemistry, Universitätsstraße 1, 40225 Düsseldorf, Germany, 2Ivoclar
Vivadent AG, Bendererstrasse 2, 9494 Schaan, Principality of
Liechtenstein and 3Heraeus Kulzer GmbH, Philipp-Reis-Straße 8,
61273 Wehrheim, Germany
doi:10.3762/bjoc.10.180
Received: 24 March 2014
Accepted: 01 July 2014
Published: 29 July 2014
Associate Editor: P. J. Skabara
Email:
Helmut Ritter* - h.ritter@uni-duesseldorf.de
© 2014 Reinelt et al; licensee Beilstein-Institut.
License and terms: see end of document.
* Corresponding author
Keywords:
composites; crosslinking; dental polymers; high performance
polymers; photopolymerization
Abstract
Thiol–ene photopolymerizations gain a growing interest in academic research. Coatings and dental restoratives are interesting appli-
cations for thiol–ene photopolymerizations due to their unique features. In most studies the relative flexible and hydrophilic ester
derivative, namely pentaerythritoltetra(3-mercaptopropionate) (PETMP), is investigated as the thiol component. Thus, in the
present study we are encouraged to investigate the performance of more hydrophobic ester-free thiol-modified bis- and trisphenol
derivatives in thiol–ene photopolymerizations. For this, six different thiol-modified bis- and trisphenol derivatives exhibiting four to
six thiol groups are synthesized via the radical addition of thioacetic acid to suitable allyl-modified precursors and subsequent
hydrolysis. Compared to PETMP better flexural strength and modulus of elasticity are achievable in thiol–ene photo-
polymerizations employing 1,3,5-triallyl-1,3,5-triazine-2,4,6-trione (TATATO) as the ene derivative. Especially, after storage in
water, the flexural strength and modulus of elasticity is twice as high compared to the PETMP reference system.
Introduction
Restorative composites consist of a polymerizable organic dominating monomers of the organic matrix used in the formu-
matrix (10–60 wt %) and inorganic fillers (40–90 wt %) [1]. Up lation of dental composites [1-4].
to date, 2,2-bis(4-(2-hydroxy-3-methacryloxyprop-1-oxy)phen-
yl)propane (Bis-GMA), 1,6-bis(methacryloxy-2-ethoxycarbony- The shrinkage stress and the leachability of unreacted
lamino)-(mixture of 2,2,4 and 2,4,4)-trimethylhexane (UDMA) monomers due to an insufficient double bond conversion are
and triethylene glycol dimethacrylate (TEGDMA) are still the two of the main drawbacks of dimethacrylate based dental
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Beilstein J. Org. Chem. 2014, 10, 1733–1740.
resins and are still a driving force for dental researches [2]. present the synthesis of various multifunctional, hydrophobic
Many efforts have been undertaken in the last decades aiming to thiol monomers all having in common a rigid phenol backbone
face and overcome these limitations [1,4-8]. For instance the and at least four primary aliphatic groups as well as the fact that
design of tailored (meth)acrylates based on calix[4]arenes they are ester-free liquids. Their behavior in thiol–ene photo-
[9,10] or tricyclodecane [11,12] backbones and use of ring- polymerizations regarding the conversion and mechanical prop-
opening monomers as cyclopropane derivatives [13,14], silo- erties before and after water storage is investigated and
ranes [15] or spiroorthocarbonates [16] are worth to be compared to the widely used PETMP. Thereby, the well-estab-
mentioned.
lished, commercially available triallyl-1,3,5-triazine-2,4,6-
trione (TATATO) is utilized as the ene monomer.
Since Sharpless et al. published their concept of click chem-
istry in 2001, these reactions gained growing impact on the Results and Discussion
design of new materials [17]. Besides the alkyne–azide reaction, Synthesis and characterization of the thiol-modified bis- and
especially thiol–ene reactions were studied extensively [18-21]. trisphenol derivatives. Most synthetic routes for the prepar-
Bowman and coworkers first investigated thiol–ene photo- ation of thiols make use of alkyl halogenide or allyl derivatives
polymerizations as promising approach towards novel, low- as suitable precursors which are subsequently converted into the
shrinkage dental restoratives [22,23]. In contrast to the chain- corresponding thiols [39]. In the present study, the synthesis of
growth mechanism of dimethylacrylate-based systems thiol–ene the phenol-based thiols (10a,b, 14a–c, 17) was accomplished by
polymerizations proceed via a step-growth mechanism [24] the radical addition of thioacetic acid to suitable allyl-modified
which offers the advantage of a delayed gel point and thus, a precursors as depicted in Scheme 1 and Scheme 2. For the syn-
reduced shrinkage stress [22,25]. These preliminary studies thesis of 10a and 10b the well-established hydroxymethylation
were an offense for further investigations in the field of thiol- of bisphenol A was the starting point of the reaction sequence
based materials covering the topics of ternary thiol–ene- and yielded compound 3. The quantitative etherification of the
methacrylate system [26-28], adaptable thiol–ene networks phenolic hydroxy groups with potassium carbonate and ethyl
[29], thiol–yne photopolymerizations [30-32], and iodide yielded the corresponding ether derivative 5. Subse-
thiol–norbornene materials [33] only to mention a few of them. quently the remaining hydroxy groups of 3 and 5 were o-alky-
However, the design of new monomers – both ene and thiol lated in the presence of a strong base such as sodium hydride
monomers – for thiol–ene polymerization is a sustained neces- and allyl bromide (6), which resulted in the formation of the
sity in order to improve the mechanical properties of the tetra-allyl (7a) and hexa-allyl (7b) derivatives (Scheme 1).
resulting networks [34]. So far most studies focused on the opti-
mization of the ene monomer while PETMP (pentaerythritol The second route for the synthesis of the allyl-modified precur-
tetra(3-mercaptopropionate)) was constantly utilized as the thiol sors (12a–c, 15) proceeded via Claisen rearrangement of the
component.
allylaryl ethers and subsequent etherification, which was
conducted in analogy to a procedure described in literature [40],
The influence of water sorption of cured dental composites on as demonstrated in Scheme 2. Bisphenol A, bisphenol S and the
the mechanical properties have been investigated and discussed trisphenol derivative 1,1,1-tris(4-hydroxyphenyl)ethane were
extensively in several studies dealing with dimethacrylate based chosen as basic structures. Subsequently, the successful radical
composites. The use of hydrophilic monomers leads to a higher addition of thioacetic acid (8) to the allyl-modified precursors
water uptake which induces reduced strength, stiffness and (7a,b, 12a–c, 15) by use of 2,2’-azobis(2-methylpropionitrile)
wear-resistance and may cause erosion of the filler or of the (AIBN) as the radical source was easily proven by the occur-
unreacted monomer [35-38]. For studies focusing on thiol–ene rence of the typical C=O valence vibration of the thioester
photopolymerizations for dental restoratives, the mechanical (9a,b, 13a–c, 16) in the corresponding FTIR spectra at
properties of the cured composites are always determined under 1684 cm−1 (Figure 1). Additionally, in the range of 6.2 to
ambient conditions. However, the mainly investigated PETMP 4.9 ppm signals corresponding with the presence of a double
monomer has a partial hydrophilic character due to its four ester bond were no longer observed in the 1H NMR spectra. Simulta-
linkages and thus, a significant influence of water sorption on neously, the appearance of a signal at 2.4 to 2.3 ppm was
the mechanical properties should be expected.
observed, which corresponds with the methyl group of the
thioester (see Figure 2).
The limited availability of rigid thiol monomers is the initial
point for this study. In analogy to Bis-GMA which is a well- The thioester derivatives 9a,b, 13a–c and 16 were successfully
established monomer for dental materials exhibiting good hydrolyzed under acidic conditions to obtain the thiols 10a,b,
mechanical properties based on its rigid structure, we wish to 14a–c and 17 as viscous oils. The corresponding FTIR spectra
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Beilstein J. Org. Chem. 2014, 10, 1733–1740.
Scheme 1: Synthetic route for the preparation of the thiol-functionalized bisphenols 10a and 10b.
showed the disappearance of the strong C=O valence vibration based thiols in photopolymerizations the widely used monomer
at 1684 cm−1 and the simultaneous appearance of the weak S–H PETMP was used as reference monomer [22,23,33]. As the
valence vibration at approximately 2560 cm−1. Exemplarily, initiating compound, the newly developed photoinitiator di(4-
Figure 1 shows a comparison of the FTIR spectra of compound methoxy)benzoyl(diethyl)germane (K69) was utilized since
12b, 13b, and 14b. In Figure 2, a typical 1H NMR spectrum of recent studies showed an excellent performance of K69 in
the thioester 13a and the thiol 14a is shown (further 1H and dental materials compared to campherchinone/4-(N,N-dimethyl-
13C NMR spectra of all thiols can be found in Supporting Infor- amino)benzoate initiation [41].
mation File 1). Besides FTIR, 1H and 13C NMR spectroscopy,
elemental analysis and mass spectrometry was conducted for As evident from Table 1, all investigated thiol derivatives (10b,
detailed characterization of all products 10a,b, 14a–c and 17. 14a–c and 17) – except for monomer 10a – outperformed the
reference monomer PETMP regarding flexural strength and
Mechanical properties of 10a,b, 14a–c and 17 employed in flexural modulus of elasticity of the photopolymerized
thiol–ene photopolymerizations. In this study we measured thiol–ene samples. For the derivatives with a propyl spacer,
the flexural strength, flexural modulus of elasticity and the 14a–c and 17, the flexural strength varied between 121.8 MPa
conversion of unfilled thiol–ene samples containing thiol com- (14a) and 112.7 MPa (17) compared to 109.3 MPa for PETMP.
pounds 10a,b, 14a–c and 17 and TATATO (see Scheme 3) in For the samples containing 14a–c and 17 no change in the flex-
stoichiometric amounts with respect to the functional groups. ural strength was observed after storing the samples in water for
For evaluation of the mechanical performance of the phenol 24 hours at 37 °C. In contrast to that, the flexural strength of the
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Beilstein J. Org. Chem. 2014, 10, 1733–1740.
Scheme 2: Synthetic route for the preparation of thiol-functionalized phenol derivatives 14a–c and 17 using the Claisen rearrangement and the
radical addition of thioacetic acid as key steps.
PETMP containing sample decreased by 50% to a value of 1.0 GPa after storage in water, whereas for 14a–c and 17 values
47.8 MPa. Similar results were found for the modulus of elas- between 3.0 GPa and 2.8 GPa were found that slightly de-
ticity: the PETMP sample showed a decrease from 3.2 GPa to creased to 2.8 GPa and 2.7 GPa after storage in water. Thus, as
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Beilstein J. Org. Chem. 2014, 10, 1733–1740.
Figure 2: Top: 1H NMR spectrum of 3,3’-(propane-2,2-diylbis(2-(3-
mercaptopropoxy)-5,1-phenylene))bis(propane-1-thiol) (14a) in CDCl3
(300 MHz, rt); bottom: 1H NMR spectrum of S,S’-((propane-2,2-
diylbis(2-(3-(acetylthio)propoxy)-5,1-phenylene))bis(propane-3,1-diyl))
diethanethioate (13a) in CDCl3 (300 MHz, rt).
Figure 1: FTIR spectra of compounds 12b, 13b and 14b.
expected, the mechanical properties of the PETMP based sam- and 17. Regarding 14a and 14b, no significant effect was found
ple were more effected by the storage in water, due to the more by changing the backbone from bisphenol A to bisphenol S.
hydrophilic character of PETMP compared to 14a–c and 17.
Monomer 10a, incorporating four thiol-groups with a longer,
more flexible and hydrophilic methoxypropyl spacer showed a
Surprisingly, no significant effect of the degree of functional- significantly lower flexural strength of 29.1 MPa, and 9.1 MPa
ization (four versus six thiol groups) on the mechanical prop- after water storage. For the flexural modulus values of 0.4 GPa
erties of the cured sample was observed while comparing 14a and 0.1 GPa, respectively, were found. The introduction of two
Table 1: Flexural strength and flexural modulus of elasticity before and after storage in H2O as well as the conversion. All samples contained a stoi-
chiometric amount of TATATO and the below-mentioned thiol, 0.5 wt % initiator (K69) and 0.04 wt % Q1301.
Flexural strength
[MPa]
Flexural modulus of elasiticity
[GPa]
Conversion
[%]
Thiol
24 h rt
24 h H2O
24 h rt
24 h H2O
Thiol
Ene
PETMP
10a
109.3 ± 2.6
29.1 ± 1.5
98.4 ± 8.5
121.8 ± 3.4
133.6 ± 6.4
127.8 ± 6.2
112.7 ± 3.1
47.8 ± 3.6
9.1 ± 0.8
3.2 ± 0.2
0.4 ± 0.1
3.0 ± 0.2
3.0 ± 0.2
2.8 ± 0.2
3.0 ± 0.2
2.9 ± 0.2
1.0 ± 0.2
0.1 ± 0.02
2.6 ± 0.3
2.9 ± 0.2
2.9 ± 0.3
2.9 ± 0.2
2.7 ± 0.1
89.2 ± 4.2
97.2 ± 4.8
81.9 ± 1.7
66.4 ± 2.7
71.4 ± 2.0
68.2 ± 1.8
76.2 ± 0.9
83.9 ± 0.9
87.2 ± 3.0
83.9 ± 2.5
93.5 ± 2.0
61.9 ± 0.8
79.0 ± 1.0
82.6 ± 0.8
10b
14a
87.9 ± 5.7
120.1 ± 2.8
132.2 ± 6.7
125 ± 12.0
119.4 ± 3.6
14b
14c
17
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Beilstein J. Org. Chem. 2014, 10, 1733–1740.
Scheme 3: Chemical structures of components utilized in the present study.
additional thiol groups in 10b compared to 10a improved the the functional groups of TATATO was investigated and
mechanical properties significantly.
compared to a reference system containing PETMP/TATATO.
The samples were prepared with a filler content of 59.98 wt %,
Additionally, the performance of derivative 17 in a dental an initiator content of 0.2 wt % and 0.02 wt % inhibitor. The
composite containing a stoichiometric amount with respect to experimental results are summarized in Table 2. For the
Table 2: Flexural strength and modulus of elasticity before and after storage in H2O, shrinkage force, exothermic setting time and exothermic setting
temperature of dental composites containing 39.8 wt % organic matrix based on stoichiometric amounts of TATATO and the thiol component (PETMP
respectively 17), 59.98 wt % inorganic filler, 0.20 wt % initiator (K69) and 0.02 wt % Q1301.
Thiol
Flexural strength 24 h
rt; (24 h H2O) [MPa]
Flexural modulus of
elasticity 24 h rt;
(24 h H2O) [GPa]
Shrinkage force
[N]
Exothermic setting
temperature (Ts)a [°C]
Exothermic setting
time (ts)b [s]
PETMP
86.9 ± 7.6;
(61.9 ± 5.7)
4.9 ± 0.2;
(2.2 ± 0.2)
48.5 ± 5.1
29.1 ± 1.2
17 ± 2.1
5.0 ± 0.01
14.0 ± 1.0
17
101.5 ± 5.8;
(97.0 ± 6.4)
5.1 ± 0.1;
(4.2 ± 0.2)
13.7 ± 0.4
aExothermic setting temperature is defined as the highest temperature reached during the curing process; bexothermic setting time is defined as the
time needed to reach the highest temperature.
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Beilstein J. Org. Chem. 2014, 10, 1733–1740.
PETMP/TATATO system values for the flexural strength of PETMP regarding the flexural strength and flexural modulus of
86.9 MPa and 61.9 MPa after storage in water while for the elasticity. To be utilizable for applications in dentistry, the
flexural modulus of 4.9 GPa and 2.2 GPa after storage in water mechanical performance after storage in water is of great
were found. For the 17/TATATO system the flexural strength importance. With respect to this aspect, the investigated thiols
was found to be 101.5 MPa slightly decreasing to 97.0 MPa lead to an improvement of the flexural strength of up to 250%
after storage in water. The corresponding flexural modulus was and up to 280% for the flexural modulus of elasticity compared
determined to 5.1 GPa and 4.2 GPa after storage in water. These to the initial values of the reference system PETMP/TATATO.
results indicate that the mechanical properties of the composite Exemplarily, the performance of monomer 17 in a filled
containing 17 and TATATO as the organic matrix are higher composite formulation was investigated and compared to a
than the reference system of PETMP/TATATO. Thus, after composite containing PETMP, still showing the excellent
water storage the sample containing 17/TATATO showed performance of the bisphenol monomer in matters of flexural
significantly improved values for the flexural strength as well as strength, modulus of elasticity and shrinkage force.
for the flexural modulus. Additionally, the shrinkage force was
measured using a Zwick UPM. For the sample containing 17/
Supporting Information
TATATO the shrinkage force (29.1 N) was found to be 40%
lower compared to the reference system PETMP/TATATO
General descriptions of materials and methods, the
(48.5 N), although the mechanical properties were even better
syntheses of the obtained compounds (5, 7a,b, 9a,b, 10a,b,
(see Table 2 and Figure 3). The exothermic temperature was
11a–c, 12a–c, 13a–c, 14a–c, 15, 16, 17) as well as
comparable for both systems. In contrast the exothermic time
analytical data (1H and 13C NMR, FTIR and EIMS
was longer for the system 17/TATATO which indicates a
analyses) of these compounds and copies of 1H and
slower polymerization kinetic (see Figure 3).
13C NMR spectra of compounds (10a,b, 14a–c, 17) plus
the 1H NMR spectra of compounds (5, 7a, 7b, 9a, 9b, 12c,
13a–c, 16) are given.
Supporting Information File 1
Experimental part.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-10-180-S1.pdf]
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